If you play Roblox on your mobile phone (Android or iPhone), here are the fixes you can use to resolve the “Can’t join a game” issue: Check if Roblox is down or not. Clear the Roblox cache.

The last way to join a specific Roblox game is if players have friends playing in that server. They are able to join any friend's game if they have the "Join In-game" setting enabled.